Effects of glucocorticoids on Fas gene expression in bovine blood neutrophils

Blood neutrophils are extremely short-lived cells that are programmed for rapid apoptosis after differentiation in bone marrow. Recently, glucocorticoids have been shown to prolong survival of human and rodent neutrophils, but the mechanisms and implications for leukocyte homeostasis and health are unclear. In this study, we investigated the effects of endogenous and exogenous glucocorticoids on Fas expression in bovine neutrophils because Fas is a major death receptor that stimulates apoptosis in circulating cells. Our study subjects were four periparturient dairy cows whose blood concentrations of cortisol peaked at calving, 15 dexamethasone-treated steers and three untreated steers whose neutrophils were exposed to dexamethasone in vitro. Fas mRNA abundance changes in collected neutrophils were monitored numerous times relative to the in vivo glucocorticoid challenges, and the relationships between these data and circulating neutrophil counts were estimated by correlation analyses. Fas mRNA and protein abundance, caspase 8 activity, and survival of neutrophils in vitro were also monitored in the presence and absence of dexamethasone. In the periparturient cows, Fas mRNA abundance in circulating neutrophils showed a sharp decrease between calving and 12 h postpartum. Based on PROC CORR analysis (SAS), this correlated negatively with blood neutrophil count (r=-0.634; P=0.0009) and serum cortisol concentration (r=-0.659; P<0.0001), but showed no relationship with serum progesterone or estradiol concentrations (P > or =0.09). Administration of dexamethasone to steers also caused a pronounced reduction in neutrophil Fas mRNA abundance that persisted for 12 h and correlated negatively with blood neutrophil count (r=-0.748; P=0.0021). In vitro, dexamethasone caused dose-dependent loss of GR proteins from the cytosol of neutrophils concurrently with Fas mRNA downregulation, which was inhibited by the glucocorticoid receptor (GR) antagonist, RU486. Dexamethasone treatment of cultured neutrophils also reduced surface Fas expression, spontaneous and sFasL-induced caspase 8 activity, and rate of apoptosis in the cells. Taken together, these in vivo and in vitro results suggest that glucocorticoids inhibit Fas expression in bovine blood neutrophils via GR activation, possibly contributing to the cells' increased longevity in culture and the pronounced neutrophilia observed in parturient cows and hormone-treated steers. We thus conclude that glucocorticoid-activated GR may change the homeostasis of circulating neutrophils, in part through its negative effects on Fas gene expression and downstream apoptosis signaling pathways.